Photometer readheads with a plurality of detectors are commonly used for quantitative chemical analysis, such as analysis of body fluids. A known quantity of body fluid sample, such as blood or urine, is placed on a test strip or in a test tube containing reagents which react with one or more quantitatively unknown body fluid components (analytes) to develop color in the analytes. Typical analytes of interest for urine include glucose, blood, bilirubin, urobilinogen, nitrite, protein, and ketone bodies. After adding color-developing reagents to urine, the foregoing analytes of interest have the following colors: glucose is bluish green; bilirubin, urobilinogen, nitrite, and ketone bodies are green; and blood and protein are red. The color developed in a particular analyte defines the characteristic discrete spectrum for absorption of light for that particular analyte. For example, the characteristic absorption spectrum for color-developed glucose falls within the upper end of the blue spectrum and the lower end of the green spectrum.
After adding reagents to develop color in the analytes of interest, an artificial source of controlled, diffuse light having a broad spectral output illuminates the test sample. The light reflected from or transmitted through the test sample is detected simultaneously by the plurality of detectors. The detectors are configured to detect different bands of wavelengths with each band containing those wavelengths which would be absorbed by one of the color-developed analytes, if present, in the test sample. That is, the spectral response of each detector encompasses the characteristic discrete spectrum of wavelengths for absorption of light of a particular color-developed analyte. The degree of absorption of light by that particular analyte is proportional to the concentration of the particular analyte in the test sample. This means that the amount of light reflected from or transmitted through the color-developed analyte to its corresponding detector is inversely proportional to the concentration of the analyte in the test sample. As a result, the concentration of the different color-developed analytes is determined by measuring the intensity of light sensed by the different detectors.
The detectors are typically silicon photodetectors having a broad band spectral response covering the range of wavelengths between 300 nm and 1100 nm. To limit the spectral responses of the silicon photodetectors to different wavelength bands, a different optical filter is positioned in front of each silicon photodetector. For example, a green optical filter is positioned in front of a first silicon photodetector, a blue optical filter is positioned in front of a second photodetector, and a red optical filter is positioned in front of a third photodetector. Thus, the silicon photodetectors are accompanied by respective optical filters with each optical filter transmitting a different band of wavelengths to its corresponding detector. Furthermore, using a specially designed housing assembly, the filter and photodetector combinations are optically isolated from each other to prevent optical crosstalk between the combinations. Optical crosstalk occurs when light passing through the optical filter and entering the photodetector of one filter and photodetector combination also enters the photodetector of another filter and photodetector combination. Since the concentration of a specific analyte is determined by the amount of light detected by the filter and photodetector combination targeting that analyte, optical crosstalk will decrease the accuracy of that determination. To optically isolate the filter and photodetector combinations from each other, the housing assembly containing these combinations includes partitions between these combinations which are impervious to light.
A drawback of using silicon photodetectors combined with optical filters to limit the spectral responses of the photodetectors is that the housing assembly containing the filter and photodetector combinations is relatively bulky. First, the housing assembly must accommodate the optical filters by securing the filters in the assembly and locating the filters in front of their respective photodetectors. Second, the housing assembly must accommodate the optically impervious partitions for optically isolating the filter and photodetector combinations from each other. A related drawback of using filter and photodetector combinations is that the component and assembly costs are relatively high. Each filter and photodetector combination is relatively expensive, and it is costly and difficult to manufacture the housing assembly for mounting these combinations and preventing optical crosstalk therebetween.
Another drawback of using filter and photodetector combinations is that the optical filters still transmit wavelengths, albeit attenuated wavelengths, outside the passband. If the spectral responses of the combinations overlap with each other, the accuracy of the measured concentrations of the different analytes targeted by the combinations is reduced. For most accurate calculations of analyte concentration, it is preferable to strictly confine the spectral responses of the photodetectors to the respective characteristic absorption bands of the color-developed analytes without the spectral responses overlapping each other.
A need therefore exists for a multi-detector readhead of a photometer for reflectance and transmittance applications which overcomes the aforementioned shortcomings associated with existing photometers using filter and silicon photodetector combinations for light detection.